1. Field of the Invention
This invention relates to control systems embodying electrical synchro signal data and, more particularly, to means for extending the linear range of the synchro angle signal information for use in control systems requiring extended control signal linearity.
2. Description of the Prior Art
In general, the control signal output of a conventional synchro device has a sinusoidal waveform which is quite linear over a limited range of about .+-.45.degree. but which becomes nonlinear from .+-.45.degree. to .+-.135.degree.. In aircraft systems, for example, the signal output of a conventional synchro is a 400 Hz carrier, the amplitude of which is modulated sinusoidally as the synchro rotor is rotated relative to the stator. Usually, this modulated 400 Hz signal is demodulated to provide a d.c. signal sinusoidally varying as a function of rotor position. In many applications only the .+-.45.degree. linear range of the synchro is used. However, in some applications it is desired to extend the linear range of the synchro output substantially beyond its normal linear range.
In the past, many techniques have been used for extending the linear range of synchro type devices. One early technique was to alter the stator and/or rotor winding distribution which proved not to be cost effective. Other techniques have been electronic, that is, by operating on the synchro output signal itself, such as by using diode/resistance networks, non-linear operational amplifiers and the like. These too have suffered from high cost and reliability problems. In U.S. Pat. No. 3,701,936, which issued on Oct. 31, 1972 to Martin et al., a composite function for extending the linear range of the sinusoidal waveform uses the function 1 + cos .theta. - sin .theta. where .theta. is synchro rotor angle.
In one application of the present invention (although not limited thereto), an extended linear range of a synchro output is most beneficial; that is, in an altitude preselect system for an aircraft flight control and/or flight director system. In such a system the present invention makes use of the sine and cosine signals from a settable synchro resolver which are indicative of altitude error, i.e. signals representative of the difference between the aircraft's existing altitude and a desired or selectable altitude, which error signal is used to cause the craft to execute an asymptotic capture of the selected altitude. For example, a pilot may select a desired altitude above or below his present altitude and establish a desired rate of ascent or descent thereto and then engage an altitude capture mode to effect the desired change in altitude and perform an asymptotic altitude capture maneuver. Automatic altitude capture limits, however, are based to a great extent on the range of linearity of the altitude error signal and the altitude rate established by the pilot. Typically, the settable altitude error is responsive to the "fine" altitude synchro output of the aircraft's air data computer which generally has a fine resolution of 5,000 feet per synchro revolution and normally would produce a substantially linear signal only over the range of plus or minus 45.degree. synchro rotation or approximately plus or minus 625 feet of altitude change. Thus, the altitude capture maneuver must be initiated at this altitude error or even below 500 feet due to tolerance build up from the desired altitude in order to use the desirable linear altitude error signal. This limited linear altitude range may not be sufficient to accomplish an altitude capture with no overshoot. For example, if a high rate of ascent or descent is established by the pilot, the 500 foot threshold of the fine altitude error synchro does not provide sufficient time, or altitude error, for the aircraft to safely pull out or push over to accomplish the asymptotic capture and an undesired overshoot results. Thus, it is desirable that the linear range of the fine synchro altitude error signal significantly be extended such that a substantial increase in the altitude capture threshold is provided to accommodate high rates of change of altitude of modern aircraft and still assure an asymptotic altitude capture.
As stated above, the prior art devices require complex relatively expensive circuits that are sometimes temperature dependent and difficult to troubleshoot, and may include external a.c. reference signals which can introduce phase shifts in the synchro output signal and non-linearity of the signal at 0.degree. which, in turn, may introduce system inaccuracies.
Accordingly, there is a need to provide a simple, inexpensive and reliable means to electronically extend the linear signal range of a conventional resolver type device.